Layout multiple Renderables into a grid.
Arg 1 is the weights for the allocation of extra horizontal space
to columns, Arg 2 is the weights for the allocation of extra
vertical space to rows, and Arg 3 is the grid of renderables to be
layed out. Each element of the grid is a tuple - the first item of
the tuple is the drawing priority. Lower priorities get drawn
first. Drawing order is significant when Renderables draw outside
their edges.